As is well known, from a functional standpoint, an IGBT type device is considered to be the equivalent of a MOSFET transistor driving a bipolar power transistor, for instance as shown in FIG. 1 of this application which is reproduced from Klaus Rischmuller's publication "PCIM90 EUROPE". Power Conversion Conference, June '90.
A description of the basics of the operation of an IGBT type of structure is given in U.S. Pat. No. 4,495,493.
A different embodiment which brings out other features of the same IGBT type device is described in U.S. Pat. No. 5,073,511.
Problems related to IGBT type devices are dealt with in Italian Patent 1241049.
From the above-referenced prior art, it can be evinced that the following are among the advantages afforded by an IGBT type device:
ruggedness, speed or relative fall time, and
conductivity modulation or relative output resistance.
Also known is that conductivity modulation is dependent on the injection of minority carriers from the P+ substrate layer, designated 10 in FIG. 1, into a resistive epitaxial layer 20 of the N- type, also shown in FIG. 1.
This lowers the device output resistance accordingly.
It follows that a reduction in conductivity modulation, resulting from a reduction in the gain of transistor 60 which is a parasitic PNP transistor intrinsic to the IGBT type device and shown in FIG. 1, will in turn result in increased output resistance of the device.
Also known is that an IGBT type device has an output resistance, or VCEsat directly tied to it, which increases with the breakdown voltage of the device.
Also known is the relationship of breakdown voltage to the thickness of the epitaxial layer and its dopant concentration. This can be expressed as follows: EQU BV=[q C X**2]/[2e] (1)
where: BV is the breakdown voltage; C is the dopant concentration in the epitaxial layer; X is the actual thickness of the epitaxial layer, i.e., the overall thickness less the thickness of the deep body layer P+; q is the electron charge; and e is the dielectric constant of silicon.
Increasing the conductivity modulation of an IGBT type device and concurrently decreasing its output resistance provides a better IGBT type device.
One viable prior approach to increasing conductivity modulation used to be a decreased doping of an N+ layer 21 which corresponds to the base of the PNP parasitic transistor 60, shown in FIG. 1, so as to turn it into a high gain transistor.
However, this involves, as the skilled one will recognize, an increase in the re-combination time of the minority carriers, and this is a variable which is inversely proportional to the device "speed".
Accordingly, this first solution is only practicable on condition that a compromise can be struck, between the increase in conductivity modulation and corresponding increase in turn-off time, which represents an improvement over the basic device.
The ruggedness of a device is defined as the device ability not to destroy itself when, during a turnoff under an inductive load, it is called upon to dissipate the power related to the product of voltage by current at the crossing point of the two curves, as shown in FIG. 2.
Also referring to FIG. 1, the parasitic thyristor of the structure are made up of the two transistors 60, of the PNP type, and 61, of the NPN type.
The triggering of this parasitic thyristor at a given current value (called the latch-up current) restricts the safe range of the device.
The gain of the transistor 60 is resolutive for the control of the parasitic thyristor triggering, which in turn provides a measure of the IGBT type device ruggedness.
As previously mentioned, gain is controlled by acting on the equivalent of the base of the PNP transistor 60 that is the layer 20 shown in FIG. 1.
It follows that, to improve the ruggedness of the IGBT type device, a transistor 60 with low gain will be aimed at, e.g., by increasing the dopant dose to the N+ layer 21.
However, this manipulation of the dopant dose will act in an inversely proportional manner on the output resistance for the reasons given above in connection with conductivity modulation.
It is evident, therefore, that any improvement of the device ruggedness would be at the expense of the other significant variable, namely conductivity modulation, and vice versa.
There is another known method of acting on the output resistance, speed and ruggedness variables without altering the breakdown voltage value.
This consists selecting the thickness of the N- layer 20.
It will be recalled that it depends on breakdown voltage according to relation (1).
The thickness of the layer 20 is known, from the physics of semiconductor electronic devices, specifically of IGBTs, to be directly proportional to the re-combination time of the minority carriers.
It follows that that thickness is inversely proportional to the device speed.
In addition, by the same laws of physics, the thickness of the layer 20 is tied in a directly proportional manner to the output resistance of the device.
In light of the foregoing, it can be evinced that the conductivity modulation can be improved, or the output resistance lowered, by decreasing the thickness of the layer 20, to concurrently improve the device speed as well.
Based on relation (1), this would result in a decreased breakdown voltage.
Also according to relation (1), it would be possible to keep the same breakdown voltage by suitably reducing the thickness of the layer 20, while increasing its conductivity by a raise in the dopant dose.
While being in many ways advantageous, the last-mentioned solution has a drawback in that it implies a considerable reduction of the resultant device ruggedness due to the strong electric field which is present at the interface between the N- layer 20 and N+ layer 21.
This is also shown by the following relation (2), which expresses the electric field in the semiconductor material as a function of the thickness of the N- layer 20: ##EQU1##
It is evinced from this relation that a decreased thickness and concurrently increased conductivity for the layer 20 is unproposable because this would seriously impair the device ruggedness.